In the related art, a transistor is known which includes a buffer layer including GaN, a barrier layer including AlGaN formed on the buffer layer, a gate insulating film disposed in a trench formed in the barrier layer, and a gate electrode facing the buffer layer with the gate insulating film interposed therebetween.
In the transistor known in the related art, the trench is formed to penetrate through an electron supply layer in order to prevent 2DEG (Two Dimensional Electron Gas) from being formed immediately under the gate electrode when the gate electrode is turned off. No 2DEG is formed in a portion in which the trench is formed since there is no boundary between the electron supply layer and an electron transit layer. Thus, a normally-off operation is realized.
However, in the transistor known in the related art, the trench is formed to dig down a portion of the electron transit layer through the electron supply layer, and a portion of a region where 2DEG is to be formed when the gate electrode is turned on is lost. Therefore, an error occurs in a control voltage required to form the 2DEG, which may result in occurrence of a switching noise or increase in undesirable energy loss.